The present invention relates to generation of fractures in lateral boreholes or drainholes and more particularly, to use of a cooling fluid to selectively form fractures at optimum locations along the lateral borehole.
Numerous oil deposits have either very high viscosity or are found in formations with very low permeability. In either case, flow rates of the oil into a conventional vertical borehole are often so low that production of the oil is uneconomical. Various techniques have been used to increase the flow of oil into the main borehole. Horizontal or lateral bores have been drilled from the main borehole hundreds of feet out into the formation. U.S. Pat. No. 3,398,804 issued to Holbert illustrates apparatus and methods for drilling such horizontal boreholes or drainholes.
Hydraulic fracturing of formations surrounding the main vertical borehole has also been used to increase flow of oil into the wellbore. It is desirable that the fractures extend as far as possible from the main borehole and that they be distributed somewhat uniformly or at least selectively throughout the formation. However, fractures form preferentially along naturally occurring stress lines and, therefore, tend to grow primarily in one plane through which the borehole passes. It has, therefore, been found desirable to use drainholes for initiating fractures at points substantially displaced from the main borehole. However, drainholes are typically not cased and it is difficult to provide sufficient pressures in isolated portions of the drainhole to selectively fracture. As a result, fractures often occur close to the main vertical borehole rather than at the desired substantial distances therefrom.
It has recently been determined that formation temperatures surrounding a wellbore affect the naturally occuring earth stresses which in turn determine pressures required for fracturing the formation. See, for example, the paper entitled "Changes in Earth Stresses Around a Wellbore Caused by Radially Symmetric Pressure and Temperature Gradients" by T. K. Perkins and J. A. Gonzalez, SPE 10080, which was presented at the Fifty-sixth Annual Fall Technical Conference and Exhibition of the Society of Petroleum Engineers, Oct. 5 through 7, 1981. In this paper, it is disclosed that changes in formation temperature caused, for example, by injection of cool water in a waterflood project over a long period of time can cause substantial reduction in earth stresses and fracturing pressure in the affected formations.